This invention relates to an image forming apparatus such as a copier, a facsimile and a printer that forms an image on a printing sheet. This invention also relates to a sheet supplying device including a sheet tray detachably attached to the image forming apparatus and a mechanism for supplying each of printing sheets stored in the sheet tray to the image forming apparatus.
There is known a sheet supplying device having a sheet placing member on which printing sheets are stacked. The sheet placing member is movable upward and downward between a sheet supplying position (in which the uppermost printing sheet contacts a pickup roller) and a retracting position. The conventional sheet supplying device has a lifter for moving the sheet placing member. The lifter is driven by a motor, and the driving force of the motor is transmitted to the lifter by means of an idle gear, a planetary gear and a lifter gear. Such a sheet supplying device is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2003-201045 (Page 6 and FIG. 7).
However, in the conventional sheet supplying device of the image forming apparatus, it is necessary to provide a lot of gears, a driving motor, a controlling circuit for controlling the driving motor and other components for moving the lifter. Thus, the number of the components may increase, and therefore the size of the sheet supplying device may become large, and the manufacturing cost of the sheet supplying device may increase.